Recovery of fluosilicic acid and p2o5 from recycle streams and pond water



R. G. sMAL'rz l-:TAL

March 3, 1970 3,498,746

RECOVERY 0F FLUOSILICIC ACID AND P205 FROM RECYCLE STREAMS AND POND WATER Filed Nov. 29, 1967 5 Sheets-Sheet 1 E2 mom@ $5 E T .Y zu@ .n umf A .O MAR smR. Y GIL LU Vl OHHA v... RWP B March 3., 19?() R. G. sMAL-rz ETAL 3,498,746

RECOVERY 0F FLUOSILICIC ACID AND P205 FROM RECYCLE STREAMS AND POND WATER 5 Sheets-Sheet 2 Filed Nov. 29, 196'? March 3, 1970 R. G. sMALTz ETAL 3,498,746

RECOVERY OF FLuosILIOIc ACID AND P O FROM REOYcLE STREAMS AND POND WATER Filed Nov. 2% 1967 f 5 Sheets-Sheet 3 no@ $3-3 E2. 5m;

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, mmhssozo March 3 1970 R. G. sMAL'rz EIAL 3,498,746

REcovERY oE ELuosILIcIo ACID .mn P o ERoM REcYcLE STREAMS AND POND WATER Filed No... ze. 19e? 5 Sheets-Sheet 4.

ROY.G. SMALTZ WILLIAM A. SATTERWHITE PAUL R. ROBERTS Marsh 3, 1970 R. G. sMALTz Erm.

RECOVERY 0F FLUOSILICIC ACID AND P O FROM RECYCLE STREAMS AND POND WATER 5 Sheets-Sheet 5 Filed Nov. 29, 196'? INVENToRs: ROY G. SMALTZ WILLIAM A. SATTERWHITE PAUL R. ROBERTS BY: t M I.

United States Patent O U.S. Cl. 23-153 6 Claims ABSTRACT OF THE DISCLOSURE Waste water from the manufacture of wet process phosphoric acid from phosphate rock and containing P205 fluoride and silica compounds is normally passed into a pond exposedto weather conditions leading to evaporation of water and the concentration of the P205 and fluoride values, and is utilized together with a recycle stream from the process which also contains P205 and fluoride values to dilute concentrated sulfuric acid so that the dilute acid containing P205 may be used in the digestion of phosphate rock for the recovery of the P205 while under the heat of dilution fluoride and silica compound vapors are evolved and condensed by contact with water and recovered as fluosilicic acid product.

BACKGROUND AND SUMMARY In the manufacture of wet process phosphoric acids and other phosphatic materials, considerable water is used in washing and other steps and waste water which has to be disposed of is passed into a large pond where it is exposed to the elements which bring about evaporation of the water and concentration of P205 and fluoride values. Not only is the pond Water considered a waste material but also it presents a hazard in that under exposure to the sun and atmospheric conditions there is some evolution of fluorine, and further, in the case of heavy rains, some of the pond water may overflow into streams.

We have discovered that pond water can be used together with recycle streams in the process which also contain P205 and fluoride values for the dilution of concentrtaed sulfuric acid to bring it to a dilute condition in which it is most effective in the digestion of phosphate rock, The P205 of the pond water and of a recycle stream is recovered while at the same time under the heat of dilution fluoride and silica compound vapors are evolved and the vapors condensed by contacting them with water and then recovered as a valuable fluosilicic acid product.

DRAWINGS In the accompanying drawings, FIG. 1 is a diagrammatic flow sheet showing the use of pond water for the dilution of sulfuric acid in the rock digestion system; FIG. 2, a diagrammatic view similar to FIG. 1, but showing the introduction of pond water into the filter system of the process and the recyclicing of Weak acid containing wash water from the filters to the dilution tank; FIG. 3, a view similar to FIG. 2, but showing the recycling of filtrate from the first filter to the dilution tank; FIG. 4, a view similar to FIG. 2 but showing the recycling of slurry from the digester to the dilution tank; and FIG. 5, a view similar to FIG. 2 but showing steps involving the concentration of the acid in evaporators.

wind brings about a concentration of P205 values, reaching as high as 0.5-2.5 percent and higher, While the fluolCe rine dissolved in the pond water increases to about 0.5- 1.0 percent. In addition, the recycle streams used in the process consisting of weak recycle acids leaving the filters after wash water is applied, strong recycle acid leaving the filters before washing, and the slurry which may be filtered or unfiltered, all contain substantial P205 Values and fluorine values. By introducing pond water into the system in place of fresh water and by using a recycle stream for diluting the concentrated sulfuric acid, it is possible to recover not only the P205, and fluorine values in the pond water, but also the additional fluorine values in the recycle stream and in one operation.

The pond water may be introduced at any point in the system. It may be introduced as wash water at the filters, or it may be introdued into the digestive system or it may be introduced at the diluter reactor. It can be introduced at any point where fresh water has heretofone been used in the system and by merging it with the recycle stream, it is possible to recover, asl stated above, the P205 and fluorine contents of the pond water.

. In the illustration given in FIG. 1, concentrated sulfuric acid (98%) and pond water are introduced into a dilution tank. We prefer to pass the pond water and some of the sulfuric acid through a pre-dilution tank, as shown in FIG. 1. The advantage of the pre-dilution step in which a fraction (10%) of the sulfuric acid is used with the pond water is that the calcium ion is precipitated on calcium sulfate crystals and the subsequent completion of the dilution is more effectively carried out. Otherwise extensive scaling and solids build-up on equipment surfaces results and shutdowns are frequent to clean out the build-up.

In the dilution tank the concentrated sulfuric acid is preferably reduced to about 25-70% sulfuric acid and the dilute acid is returned to the digestion step for the digestion of phosphate rock. Best results have been obtained when the sulfuric acid is diluted to about 40% acid. The dilute sulfuric acid carries with it the P205 values of the pond water and these values are carried directly into the phosphate rock digesters. The digested material which preferably contains about 31-32% P205, but which may contain a wide range of P205 values, is passed to filters and washed with water, a weak recycle stream being returned to the digesters as shown in FIG. 1. The wash water may consist of pond water or water from any source. Weak acid from the lters may be passed to evaporators and a concentrated acid such as, for example, 54% P205 acid, recovered as product.

The heat of dilution in the dilution tank brings about an evolution of fluoride and silica compound vapors which may be passed through a packed column and used to strip more fluorine from other plant streams and thence on to scrubbers where they are condensed kwith water and thus form fluosilicic acid which is recovered as product. If desired, the water containing the condensed fluoride and silica vapors may be filtered to remove SiO2. For final scrubbing of the gases, water is preferably introduced through a packed column and flows counter-current to the flow of gases and a vacuum is maintained upon the column by a barometric condenser as shown in conventional apparatus illustrated in FIG. 1. If desired, other phosphoric acid streams may be passed down the packed column, as shown in FIG. l, defiuorinated acid being recovered from the bottom of the packed tower and passed to a surge tank and from thence to evaporators, as also shown in FIG. l.

In the illustration given in FIG. 2, the operation is as shown in FIG. 1 except that the pond water is introduced as wash water in the filter system and the weak filtrate is recycled to the pre-dilution (indicated as 10% H250., Tank) tank where it meets a portion of the sulfuric acid, the combined material being passed into the dilution tank indicated on the drawing as 40% H2SO4 Dilution Tank. 5

In the voperation shown in FIG. 3, pond water is also introduced into the filter system and the recycle stream to the 10% H2804 Tank is the filtrate directly leaving the first filter and containing a substantial amount of P205 as, for example, 30% P205.

In the operation shown in FIG. 4, pond Water is introduced as Wash Water in the filter system and slurry is passed from the digestion system to the pre-dilution acid tank. Otherwise the operation is as described above in connection with FIG. l.

FIG. 5 shows an operation which is similar to that described in FIG. 2 but shows in addition the concentration of the 54% P205 acid to 60% P205 acid. The figures 54% P205 and 60 P205 are merely illustrative of lower and higher concentrated acids which in practice may have a relatively wide range. In the concentration of the acid, a barometric condenser indicated by the numeral 20 is employed to maintain the concentrator under very high vacuum. We find it advantageous to introduce the concentrated acid (98%) into the barometric condenser Where it meets the steam and iluorine vapors leaving the phosphoric acid concentrating vessel. In this operation the concentrated sulfuric acid may be diluted to about 94% acid by condensing steam and the 94% acid may be returned, as shown in FIG. 5, to the H2S04 Tank and the 40% H2504 Dilution Tank.

In all of the foregoing operations described in FIGS. l-5 inclusive, pond water may be employed to replace fresh water used in the lsystem and may `be added at any places where heretofore fresh Water was used. The concentrated acid may be diluted from about 98% to about 25-70% in the dilution tank. If desired, the starting sulfuric acid may be 93-40% sulfuric acid, but We prefer to start with an acid in the range of about 94-98% Specific examples illustrative of the invention may be set out as follows.

Example I Dilution of 98% sulfuric acid to various concentrations with weak filtrate (recycle) phosphoric acid was completed at atmospheric pressure. The dilutions were completed in an agitated beaker sitting in a heating mantel. The purpose of the heating mantel was to maintain the solution at its boiling temperature for thirty minutes retention. Tests were completed with the following acids:

Percent Percent Percent P 5 F S O 4 Weak filtrate 22. 86 1. 90 2. 56 Sulfurie acid 98. 13

The following results were obtained:

Final solutions Approximate Percent Percent Percent Evolution, dll. 0f H2504 P205 O4 F 0f F *Calculated from percent F and percent P205 analyses.

Example II Dilution of 98% sulfuric acid -to 40% sulfuric acid with weak filtrate (recycle) phosphoric acid was completed at 6 inches of mercury absolute pressure. The retention time was varied and the lluorine evolution efficiencies were determined.

The dilutions were made with batchwise additions of the sulfuric acid and. weak filtrate phosphoric acid 4 into an insulated aspirator bottle with agitation. Acids of the following analyses were used for the test:

Percent Percent Percent P O F Weak filtrate 23. 73 2. 22 2. 71 Sulfurio Acid 96. 5

The following results were obtained:

Final solutions Retention time, Percent Percent Percent Evolution; minutes P205 O4 F of F *Calculated from percent F and percent P205 analyses.

Higher percentages of fluorine evolution would be anticipated in plant scale facilities where better heat conservation and stream controls would be practiced. Also higher evolution may be achieved through supplemental stream sparging.

While in the foregoing specification, we have set out specific procedure in considerable detail for the purpose of illustrating embodiments of the invention, it will be understood that such details may be varied widely by those skilled in the lart without departing from the spirit of our invention.

We claim:

1. In a phosphate rock treating system requiring the addition of water and in which phosphate rock is digested in a digestion zone with sulfuric acid, and the digested mixture is filtered, and washed, producing ltrate and slurry streams, at least one of said streams being recycled to the digestion zone, the steps of supplying pond water containing P205 and fluoride values to said system to merge with said recycle stream, and diluting concentrated sulfuric acid with said recycle stream whereby under the heat of dilution fluoride vapors are evolved and subsequently condensed by contact with Water and recovered as fluosilicic acid product.

2. The process of claim 1 in which said pond water is used as wash water in the ltering step, and the ltrate containing pond water is used to dilute the concentrated sulfuric acid and the dilute acid recycled to the digestion zone.

3. The process of claim 1 in Which the -bulk of the sulfuric acid is diluted in one dilution zone and in which 'a small portion of the concentrated sulfuric acid is prediluted with water selected from the group consisting of pond water and said recycle stream to precipitate calcium sulfate therein.

4. The process of claim 1 in which phosphoric acid product is withdrawn from the system and concentrated under reduced pressure maintained by a barometric condenser and in which the concentrated sulfuric acid is introduced into said barometric condenser in contact with vapors from the concentration zone to dilute the sulfuric acid and the diluted acid then contacted with said recycle stream.

5. In a process in which phosphate rock s digested with sulfuric acid in a digestion Zone and then the digested mixture is filtered and Washed producing ltrate and slurry streams, and at least one of said stream is recycled to said digestion zone, the steps of continuously Iadding pond Water containing P205 and fluoride values as wash Water during the step of filtration and then diluting concentrated sulfuric acid with said recycle stream to recover P205 in said dilute sulfuric acid, while under the heat of dilution fluoride vapors are evolved, condensing said vapors =by contact with water, and recovering lluosilicicv acid as product.

6. The process of claim 1 wherein said concentrated 5 6 sulfuric acid is diluted to about 25% to 70% H2504 With 3,256,061 6/1966 Tufts et al. 23-153 said recycle stream. 3,273,713 9/ 1966 Parish 23-153 References Cited OTHER REFERENCES Smiths College Chemistry, 7th Ed., 1960, p. 505. UNITED STATES PATENTS 5 Appleton-Century-Crofts, Inc., New York. 1,938,533 12/1933 Peniield 23-153 2,636,806 4/ 1953 Winter 23--153 XR EDWARD STERN, Primary Examiner 2,962,357 11/1960 Williams et al 23165 3,091,513 5/1963 Parish 23-153 U.S. Cl. X.R.

3,151,941 10/1964 Hollingsworth et al. 23-165 10 23-122, 165, 182

Disclaimer 3,498,746.R0y G. Smaltz, Valrico, William A. Sattemvhte, Lakeland, and Paal R. Roberts, Plant City, F1a.. RECOVERY OF FLUOSILICIC ACID AND P205 FROM RECYCLE STREAMS AND POND WATER. Patent dated Mar. 3, 1970. Disclaimer filed June 24, 1977, by the assignee, United States Steel Corporation. Hereby enters this disclaimer to all of the claims therein of said patent.

[Oc'ial Gazette August 23, 1.977.] 

